54 LAND AND FRESH-WATER SHELLS. 



THE EXCRETORY ORGANS. The excretory organs, organs of 

 Bojanus, or nephridia\ are situated just beneath the pericardium. 

 They excrete guanin, not uric acid. Each is composed of a 

 glandular part, communicating in front with the pericardial cavity, 

 and of a non-glandular muscular part or vestibule, which communi- 

 cates with the fellow of the opposite side by the inter-renal aperture, 

 and with the exterior by a small pore the external renal aperture 



Fig. 20. The Organ of Bojanus in diagrammatic plan. 



between the two lamellae of the inner gill. The glandular 

 portion consists of numerous lamellae, and on these Koll- 

 mann has described blind ciliated funnel-shaped openings 

 which may in Anodonta be present to the number of two 

 hundred. 



THE NERVOUS SYSTEM. The nervous system consists of three 

 pairs of ganglia the cerebral or supra- cesophageal (which according 

 to Sprengel represents the cerebral plus the pleural ganglia in the 

 snail), the pedal, and the osphradial, olfactory, or paries-splanchnic 

 ganglia, with numerous commissures. The cerebral ganglia are 

 situated at the base of the labial palps, and are united together 

 over the mouth by an inter-cerebral commissure ; they are triangular 

 in shape, and supply branches to the palps, mantle, gills, and 

 neighbouring muscles. From these ganglia t\vo cords, forming 

 the cerebro-pedal commissures, run downwards and backwards to 

 l\\t pedal ganglia in the foot. The pedal ganglia are situated at 

 the junction of the visceral mass and the foot ; they are orange- 

 coloured bodies giving branches to the foot. It is generally 

 stated that the nerve to the otocyst coraes from the pedal ganglia, 

 but according to the later researches of Simroth, it must be con- 

 sidered as arising from the cerebro-pedal commissure. 



